'Believing with you that religion is a matter which lies solely between man and his God, that he owes to none other for his faith or his worship, that the legislative powers of government reach actions only, and not opinions, I contemplate with solemn reverence that act of the whole American people which declared that their legislature should 'make no law respecting an establishment of religion, or prohibiting the free exercise thereof,' thus building a wall of separation between Church and State.'
-- Thomas Jefferson
Religious liberty is one of America's founding principles. The Constitution does not endorse any religious creed, and it does not recognize any power of government to decide theological questions. The right to practice religion, or no religion at all, is among the most fundamental of the freedoms guaranteed by the Bill of Rights.
The First Amendment contains two explicit provisions concerning religion. The Establishment Clause, 'Congress shall make no law respecting an establishment of religion...' was written to make sure that the government would not promote, endorse, finance or encourage any particular religious beliefs or symbols. The 'Free Exercise Clause' of the First Amendment guarantees the right to practice one's religion free of government interference.
The ACLU has worked since its inception to ensure that religion remains the business of individuals, families, and religious communities, and that Americans have the right to demonstrate their religious beliefs in public and private.
The Establishment Clause of the First Amendment was almost entirely ignored in the realm of public education in the United States for over a century. At the beginning of the Twentieth Century, as Darwinian principles were increasingly taught in the public school system, Christian fundamentalists mounted a concerted effort to prohibit the teaching of any theory about the origins of human life that conflicted with their interpretation of the Bible. They equated evolution with 'modernist ideas' that 'undermined American values' and many states across the South enacted laws that banned the teaching of evolution in schools. In 1924, the ACLU released a study which found more unconstitutional teaching laws on the books than at any other time in American history: laws that required Bible reading and religious study in public school, barred pacifists from teaching, and proscribed the teaching of evolution.